Pentafoil Knot

Figure 10.89 Conceptual production of increasingly interlocked species through the use of multime-tallic helicates. (a) Doubly interlocked [2]catenane from three metal centres, (b) Pentafoil knot from four metal ions, (c) Triply interlocked [2]catenane. (Reproduced with permission from [98]).

Their strategy was successful and they were able to isolate the pentafoil knot in 44% yield. In addition to the metal-assisted templation of the strands to form a circular helicate precursor, the chloride anion was shown to assist the formation of the pentanuclear adduct. The use of 3,6-dioxo-l,8-diaminooctane allowed the knots to be closed while simple aliphatic diamines failed to produce a clean closing procedure. The synergy of metal and anion coordinations was employed simultaneously to template the formation of a pentafoil knot structure. 

SCHEME 17.8 Metal-assisted synthesis of pentafoil knot of Leigh et al. and its crystal structure. Fe atoms are shown as dark grey halls, chloride anion as light gray. H atoms are omitted for clarity (adapted from CCDC 845599). 

It needs to be noted that a trefoil knot is only the first member of a potential series of related molecular knots.If the number of metal centres employed in generating the associated helix is even, then the number of crossing points will be odd, and a single, closed knotted structure will be formed (trefoil, pentafoil, heptafoil, etc.). On the other hand, if the number of metal centres is odd, then an even number of crossing points will result, giving rise to multiply-interlocked [2]-catenanes. 

A strategy of using polynuclear double helices as precursors to more complex interlocked and knotted systems should allow the carrying out of template synthesis of trefoil, pentafoil, heptafoil... knots containing a set of two, four, six. .. metals, respectively. 

FIGURE 17.1 Most common knots and links and theirmathematical descriptions. Left to right [2]catenane, trefoil knot, Solomon link, pentafoil (or Solomon s seal) knot, and Borromean ring. 

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